Methods for electroplating articles with metal coatings generally involve passing a current between two electrodes in a plating solution where one of the electrodes is the article to be plated. A typical acid copper plating solution comprises dissolved copper, usually copper sulfate, an acid electrolyte such as sulfuric acid in an amount sufficient to impart conductivity to the bath, and proprietary additives to improve the uniformity of the plating and the quality of the metal deposit. Such additives include accelerators, levelers, and suppressors, among others.
Electrolytic copper plating solutions are used in a variety of industrial applications, such as decorative and anticorrosion coatings, as well as in the electronics industry, particularly for the fabrication of printed circuit boards and semiconductors. For circuit board fabrication, copper is electroplated over selected portions of the surface of a printed circuit board, into blind vias and onto the walls of through-holes passing between the surfaces of the circuit board base material. The walls of a through-hole are first made conductive, such as by electroless metal deposition, before copper is electroplated onto the walls of the through-hole. Plated through-holes provide a conductive pathway from one board surface to the other. For semiconductor fabrication, copper is electroplated over a surface of a wafer containing a variety of features such as vias, trenches or combinations thereof. The vias and trenches are metallized to provide conductivity between various layers of the semiconductor device.
It is well known in certain areas of plating, such as in electroplating of printed circuit boards (“PCBs”), that the use of accelerators and/or levelers in the electroplating bath can be crucial in achieving a uniform metal deposit on a substrate surface. Plating a substrate having irregular topography can pose particular difficulties. During electroplating a voltage drop variation typically exists along an irregular surface which can result in an uneven metal deposit. Plating irregularities are exacerbated where the voltage drop variation is relatively extreme, that is, where the surface irregularity is substantial. As a result, a thicker metal deposit, termed overplating, is observed over such surface irregularities. Consequently, a metal layer of substantially uniform thickness is frequently a challenging step in the manufacture of electronic devices. Leveling agents are often used in copper plating baths to provide substantially uniform, or level, copper layers in electronic devices.
The trend of portability combined with increased functionality of electronic devices has driven the miniaturization of PCBs. Conventional multilayer PCBs with through-hole interconnect vias are not always a practical solution. Alternative approaches for high density interconnects have been developed, such as sequential build up technologies, which utilize blind vias. One of the objectives in processes that use blind vias is the maximizing of via filling while minimizing thickness variation in the copper deposit across the substrate surface. This is particularly challenging when the PCB contains both through-holes and blind vias.
Generally, leveling agents used in copper plating baths provide better leveling of the deposit across the substrate surface but tend to compromise the throwing power of the electroplating bath. Throwing power is defined as the ratio of the hole center copper deposit thickness to its thickness at the surface. Newer PCBs are being manufactured that contain both through-holes and blind vias. Current bath additives, in particular current leveling agents, do not provide level copper deposits on the substrate surface and fill through-holes and/or fill blind vias effectively. Accordingly, there remains a need in the art for leveling agents for use in copper electroplating baths used in the manufacture of PCBs that provide level copper deposits while not significantly affecting the throwing power of the bath.